Loud tap changer by-pass switch contact assembly and material composition thereof

ABSTRACT

Contact structures with composite materials arranged to increase the operating life of the contact when used in load tap changer by-pass switches. An arc resistance refractory material is dimensioned and positioned to interrupt the current when the switch is opened. A highly conductive metallic material forms a sliding portion of the contact structure and is dimensioned and positioned to conduct all of the switch current while the rotary blade of the switch is rotating. The material of the sliding portion prevents galling of the rotary blade and considerably extends the operating life of the by-pass switch.

United States Patent [1 1 Topper 1 June 26, 1973 LOUD TAP CHANGER BY-PASS SWITCH CONTROL ASSEMBLY AND MATERIAL COMPOSITION THEREOF [75] Inventor: Thomas B. Topper, West Middlesex,

[21] Appl. No.: 185,161

3,166,660 l/l965 Gribble 200/166 C 3,226,517 12/1965 Schreinder 200/166 C 2,717,296 9/1955 Foley et a1. 200/166 C 3,192,328 6/1965 Wilson, Jr. 200/11 B Primary Examiner-J. R. Scott Attorney-A. T. Stratton and F. E. Browder [57] ABSTRACT Contact structures with composite materials arranged to increase the operating life of the contact when used in load tap changer by-pass switches. An arc resistance 2 F' zoo/166 hai refractory material is dimensioned and positioned to 'F' B 4 interrupt the current when the switch is opened. A 1 1e 0 care l 1 6 66 highly conductive metallic material forms a sliding portion of the contact structure and is dimensioned and 56 R f d positioned to conduct all of the switch current while 1 e erences the rotary blade of the switch is rotating. The material UNITED STATES PATENTS of the sliding portion prevents galling of the rotary 2,931,876 4/1960 Weinfurt ZOO/166 C UX blade and considerably extends the operating life of the 1,978,516 10/1934 Weiger et a1 200/166 C UX by-pass switch. 2,464,591 3/1949 Larsen et al. 200/166 C X 3,140,373 7/1964 Horn ZOO/166 C X 10 Claims, 5 Drawing Figures @5 =2 :e I IS Q/HZ Q :2 86 m 94 104 g 1 LOUD TAP CHANGER BY-PASS SWITCH CONTROL ASSEMBLY AND MATERIAL COMPOSITION THEREOF BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates, in general, to power transformer tap changers and, more specifically, to by-pass switch contacts used in vacuum-type load tap changers.

2. Description of the Prior Art Tap changers are used in conjunction with high power transformers to change the turns ratio of the windings thereof and thus change the voltage and/or current output of the transformer. Load-type tap changers are more complex than no load-type tap changers because of the necessity of maintaining the output of the transformer during a tap changing sequence. A load tap changer utilizing a selector switch, a vacuum switch and a by-pass switch is disclosed by US. Pat. No. 3,553,395, issued on Jan. 5, 1971 and assigned to the same assignee as this invention.

The tap changer disclosed in US. Pat. No. 3,553,395 uses a by-pass switch to divert the load current around the vacuum switch when the tap changer is not changing taps. This permits more efficient use of the vacuum switch and thus the maintenance free life of the tap changer is enhanced. However, since relatively large currents usually flow through the by-pass switch, the contacts of the bypass switch must be capable of interrupting large currents for a relatively large number of times. Vacuum switches of the type used to interrupt the load current have a contact life in excess of 1.5 million operations at a current of 2,000 amperes. By-pass switch contacts are not presently available which give this excellent performance. In fact, the number of maintenance free operations which can be expected from the overall tap changer is substantially limited by the life expectancy of the by-pass switch contacts.

The operation of the by-pass switch requires that each stationary contact be capable of carrying large currents while sliding on a rotary blade which is normally constructed of high conductivity copper alloy. It is also a requirement that the contacts be constructed of a suitable material which will withstand the destructive effects of an electrical are which develops when the current is interrupted. Unfortunately, good are resisting materials do not provide suitable contact surfaces for the sliding portion of the contact. As a result of the poor performance of the contact material used in prior art by-pass switch contacts, galling or commutation arcing occurs between the contacts and the rotary blade. This pits the surface of the rotary blade which slides along the contacts and, when the pitting is severe enough, the switch becomes inoperative. Even in the liquid coolant of the tap changer the bypass contact life at 2,000 amperes may be as low as 50,000 operations.

Therefore, it is desirable, and it is an object of this invention, to provide a by-pass switch having contacts which will increase the maintenance free life of the bypass switch.

SUMMARY OF THE INVENTION There has been disclosed a load tap changer by-pass switch having a contact structure which substantially increases the life of the by-pass switch over prior art bypass switches.

In one embodiment of the invention, the stationary contact structure comprises a body portion, a current interrupting portion attached thereto and constructed of an are resistant refractory material, and a sliding portion constructed of a highly conductive metallic material which is attached to a face of the current interrupting portion. The sliding portion is the only part of the contact structure which touches the rotary blade of the by-pass switch when it is being rotated. The current interrupting portion touches the rotary blade momentarily when the switch is opened and the are which develops occurs between the rotary blade and the current interrupting portion of the contact structure.

In another embodiment of the invention, the sliding portion is attached directly to the body portion of the contact structure. The current interrupting portion, which is attached to the body portion adjacent to the sliding portion, has the face, which is adjacent to the rotary blade, beveled so that the current interrupting portion does not touch the rotary blade except when the switch is being opened. While sliding, only the sliding portion touches the rotary blade.

By using the composite contact structure disclosed herein, the performance of the by-pass switch has been increased considerably. While by-pass contact life ac cording to the prior art has been approximately 50,000 operations, the new and useful structure disclosed by this invention has increased the by-pass switch contact life to over 1.5 million operations.

BRIEF DESCRIPTION OF THE DRAWINGS Further advantages and uses of this invention will become more apparent when considered in view of the following detailed description and drawings, in which:

FIG. 1 is an electrical schematic of a load-type tap changer constructed according to the teachings of this invention;

FIG. 2 is a view of a by-pass switch constructed according to the teachings of this invention;

FIG. 3 is a cross-sectional elevational view of the bypass switch contact structure taken along the line III- -lll,shown in FIG. 2;

FIG. 4 is a cross-sectional view of the contact structure taken along the line IV-IV of FIG. 3 illustrating one embodiment of the invention; and

FIG. 5 is a cross-sectional view of a contact structure constructed according to the teachings of another embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Throughout the following description, similar reference characters refer to similar members in all figures of the drawings.

Referring now to the drawings, and FIG. I in particu lar, there is shown a schematic diagram of a load tap changer which utilizes the teachings of this invention. As illustrated in FIG. I, the tap changer is connected to the windings 12, 14 and 16 of an electrical transformer. The transformer may be single phase or polyphase, and of the autotransformer or isolated winding type, with only a portion of a single phase being illustrated in FIG. 1 since other phases would be similarly arranged.

The tap changer is of the type which includes a no load-type tap selector switch 20, having a plurality of stationary contacts C1, C2, C3, C4, C5, C6, C7 and C8 connected to the taps T1, T2, T3, T4, T5, T6, T7 and T8, respectively, on the winding 14, and a stationary contact C9 connected to the winding 16. The tap selector switch has a pair of movable contact arms 22 and 24 for selectively and sequentially moving between the spaced stationary contacts Cl through C9. The ends of the tapped winding 14 are connected to the stationary contacts 26 and 28 of a reversing switch 30, which has a movable contact 32 connected to the winding 16, and thus to the stationary contact C9of the tap selector switch 20. The reversing switch 30 may be actuated to change its movable contact 32 from one stationary contact to the other when one of the movable contact arms 22 and 24 of the tap selector switch 20 is in engagement with the stationary contact C9 and the other arm is in transition to or from the contact C9. This allows the tapped winding voltage to be added to or subtracted from the voltage of the windings 12 and 16, depending upon the position of the reversing switch 30. In order to enable the movable contact arms 22 and 24 to be connected to adjacent taps, and thus bridge a portion of the winding 14 and enable the tap changer system to operate continuously in the bridging position to obtain a voltage half-way between the voltage of the two adjacent taps, the contact arms are connected to the winding 12 through a split or divided preventive autotransformer or reactor 40 having windings 42 and 44 disposed on a common magnetic core 46. The windings are wound to present a high impedance to circulating currents, while providing very little impedance to power current flow in the same direction through the two windings.

A single arcing duty, normally closed vacuum switch 50, and a by-pass switch 52, completes the tap changer, with the by-pass switch 52 having first and second stationary contacts 54 and 56, and a movable contact 58. The movable contact 58 is connected to the winding 12, and the stationary contacts 54 and 56 are connected to the winding sections 42 and 44 of the reactor 40. The movable contact 58 is arranged to engage both stationary contacts 54 and 56, or to select either of the stationary contacts individually. The vacuum switch 50 has contacts 62 and 64 disposed within an evacuated envelope, with one of the contacts being movable relative to the other by way of a bellows which maintains the vacuum seal. The vacuum switch 50 is connected across the contacts 54 and 56 of the by-pass switch 52. When the tap-changer is in a steady state position, the power circuit of the transformer includes the winding 16, and the portion of the winding 14 which is between the closed positions of the reversing switch 30 and the tap selected by the contact arms 22 and 24 through the two branch circuits of the contact arms. One of the branch circuits includes the contact arm 24, the winding section 42, and the portion of the by-pass switch 52 which includes the stationary contact 54. The other branch circuit includes the contact arm 22, the winding section 44, and the portion of the by-pass switch 52 which includes the stationary contact 56. The branch circuits combine in the movable contact 58 of the bypass switch 52, and the power circuit continues to the winding 12. Instead of having the tapped winding 14 connected between the two windings of a transformer, it may also be disposed at either end of a main transformer winding,

The vacuum switch 50 has its contact closed, but since it is normally shorted by the by-pass switch 50,

there is negligible current flow therethrough. Therefore, the contacts of the vacuum switch are not heated by the current flowing in the transformer winding. The tap changer drive means sequentially operates the by-pass switch 52, the vacuum switch S0, and the tapped selector switch 20, by suitable mechanical means, generally indicated by the dashed lines 72, 74 and 76, respectively. A detailed description of the operation of the tap changer is included in U.S. Pat No. 3,553,395, issued on Jan. 5, 197i and assigned to the same assignee as this invention. i

The by-pass switch 52 is illustrated showing the mechanical details of its construction in FIG. 2'. The movable contact structure 58, which is shown schematically in FIG. 1, comprises the stationary contact structures and 82 and the rotary blade 84. The rotary blade is alternately rotated clockwise and counterclockwise by the mechanical drive means 72 to engage and disengage the rotary blade 84 with the stationary contact structures 86 and 88. When the rotary blade 84 is being rotated, the current flowing between the stationary contacts 80, 82, 86 and 88 crosses the sliding junction between the stationary contacts 86 and 88 and the rotary blade. If the rotary blade 84 is rotating in a counterclockwise direction, an arc will occur when the edge 90 of the rotary blade 84 disengages with the stationary contact structure 86. A similar arcing takes place between the stationary contact structure 88 and the rotary blade edge 92 when the rotary blade 84 is rotated in clockwise direction to open the switch 52. The rotary blade 84 may be constructed of a highly conductive material, such as copper-cadmium, with the edges 90 and 92 thereof constructed of an are resistant refractory material, such as copper-tungsten.

FIG. 3 is a cross-sectional view of the stationary contact structure 86 taken along the line Ill-1II shown in FIG. 2. The stationary contact structure 86 includes the upper and lower body portions 94 and 96, respectively, which are constructed of a suitable material, such as copper. A suitable material, such as coin silver, is attached to the end of the body portions 94 and 96 to provide the structures 98 and 100 which engage with the mounting plate 102. The upper body portion is attached to a current interrupting portion 104 which has a sliding portion 106 attached thereto. The current interrupting portion 108 and the sliding portion 110 are similarly disposed on the lower body portion 96. The bolt 112 extends through holes in the body portions 94 and 96 and, with the nut 114 and the spring 116, pull the body portions together to make satisfactory electrical contact with the rotary blade 84 and'the mounting plate 102.

A partial cross-sectional view of the lower contact structure, taken along the line IV-IV of FIG. 3, is illustrated in FIG. 4. The current interrupting portion 108 is attached to the body portion 96 by a suitable process, such as silver soldering. The current interrupting portion 108 is constructed of an are resistant refractory material, such as copper-tungsten. The sliding portion 110 comprises a layer of highly conductive metallic material, such as coin silver, and is attached to the current interrupting portion 108 by a suitable process, such as silver soldering. The current flowing between the body portion 96 and the rotary blade 84 must pass across the junction 113 where the sliding portion 110 and the rotary blade 84 meet.- The material of the sliding portion 110 allows the current to flow, while the rotary blade 84 is being rotated, without substantial galling or commutation-arcing occurring between the surfaces comprising the junction 113. When the end of the blade is reached while rotating in the direction indicated by the arrow 115, the are resistant refractory material of the current interrupting portion 108 carries the main force of the are which occurs on opening the electrical circuit. Because of the high melting temperatures of the current interrupting portion 108, the surface erosion thereof is relatively small. The side 117 of the current interrupting portion 108 is beveled to allow the rotary blade 84 to engage with the contact structure.

The structure illustrated in FIG. 5 represents another embodiment of this invention. The body portion 96 is attached directly to the current interrupting portion 120 and to the sliding portion 122 by a suitable process, such as silver soldering. As the rotary blade 84 rotates, current flows across the junction 124 which is created by the rotary blade 84 and the sliding portion 122. This embodiment of the invention reduces galling or commutation-arcing" because of the characteristics of the materials which slide with respect to one another. When the rotary blade 84 rotates in a direction indicated by the arrow 126 to open the switch, an arc occurs between the rotary blade 84 and the current interrupting portion 120. The face portion 126 of the current interrupting portion 120 is beveled to prevent contact with the rotary blade 84 when the rotary blade is sliding across the sliding portion 122. The face 128 of the sliding portion 122 is also beveled slightly beyond the junction of the current interrupting portion 120 and the sliding portion 122 to assure that the current interrupting portion 120 does not contact the rotary blade 84 while sliding over the sliding portion 122. A bevel angle 130 of 5 is satisfactory. The side 132 of the current interrupting portion 120 is beveled to permit the rotary blade 84 to engage with the contact structure. The contact structures disclosed herein permit a substantial increase in the operating life of load-type tap changers using by-pass switches. Since numerous changes may be made in the above described apparatus and different embodiments of the invention may be made without departing from the spirit thereof, it is intended that all of the matter contained in the foregoing description, or shown in the accompanying drawings, shall be interpreted as illustrative rather than limiting.

I claim as my invention: 1. A by-pass switch comprising a rotary blade and stationary contact structures, said rotary blade being.

movable to contact said stationary contact structures to close said switch and movable to disengage said stationary contact structures to open said switch, each of said contact structures having a body portion, a current interrupting portion constructed of an arc resistant refractory material, and a sliding portion constructed of a highly conductive metallic material, said sliding por tion being positioned adjacent to said current interrupting portion to provide the only current path between said rotary blade and said current interrupting portion when the switch is in a closed position, with any arcing, when the switch is being opened, occurring substan' tially between said rotary blade and said current interrupting portion.

2. The by-pass switch of claim 1 wherein the body portion is constructed of copper and the highly conductive metallic material comprises silver.

3. The by-pass switch of claim 1 wherein the are resistant refractory material comprises copper-tungsten.

4. The by-pass switch of claim 1 wherein the current interrupting portion comprises first and second faces, first and second sides, said first face being attached to the body portion of the contact structure, the second face being attached to the sliding portion of the contact structure, with the sliding portion covering substantially the entire surface of said second face.

5. The by-pass switch of claim 4 wherein the sliding portion comprises a layer of highly conductive metallic material with a thickness of approximately one thirtysecond of an inch. 7

6. The by-pass switch of claim 4 wherein the first side of the current interrupting portion is beveled.

7. The by-pass switch of claim 1 wherein the current interrupting portion comprises first and second faces and first and second sides, the sliding portion comprises first and second faces and first and second sides, said first face of the current interrupting portion being attached to the body portion of the contact structure, said first face of the sliding portion being attached to the body portion of the contact structure, said second side of the current interrupting portion being adjacent to the first side of the sliding portion, said second face of the sliding portion being a greater distance from the body member than said second face of the current interrupting portion, said second face of the current interrupting portion being sloped, with respect to the second face of the sliding portion, to prevent contact of the current interrupting portion with the rotary blade when the switch is in a closed position.

8. The by-pass switch of claim 7 wherein the first side of the current interrupting portion is beveled.

9. The by-pass switch of claim 7 wherein an edge of the second face of the sliding portion is beveled and aligned with the slope of the second face of the current interrupting portion.

10. The by-pass switch of claim 9 wherein the second face of the current interrupting portion and the beveled edge of the sliding pot-tion has a slope of approximately 5 with respect to the second face of the sliding portion of the contact structure. 

2. The by-pass switch of claim 1 wherein the body portion is constructed of copper and the highly conductive metallic material comprises silver.
 3. The by-pass switch of claim 1 wherein the arc resistant refractory material comprises copper-tungsten.
 4. The by-pass switch of claim 1 wherein the current interrupting portion comprises first and second faces, first and second sides, said first face being attached to the body portion of the contact structure, the second face being attached to the sliding portion of the contact structure, with the sliding portion covering substantially the entire surface of said second face.
 5. The by-pass switch of claim 4 wherein the sliding portion comprises a layer of highly conductive metallic material with a thickness of approximately one thirty-second of an inch.
 6. The by-pass switch of claim 4 wherein the first side of the current interrupting portion is beveled.
 7. The by-pass switch of claim 1 wherein the current interrupting portion comprises first and second faces and first and second sides, the sliding portion comprises first and second faces and first and second sides, said first face of the current interrupting portion being attached to the body portion of the contact structure, said first face of the sliding portion being attached to the body portion of the contact structure, said second side of the current interrupting portion being adjacent to the first side of the sliding portion, said second face of the sliding portion being a greater distance from the body member than said second face of the current interrupting portion, said second face of the current interrupting portion beiNg sloped, with respect to the second face of the sliding portion, to prevent contact of the current interrupting portion with the rotary blade when the switch is in a closed position.
 8. The by-pass switch of claim 7 wherein the first side of the current interrupting portion is beveled.
 9. The by-pass switch of claim 7 wherein an edge of the second face of the sliding portion is beveled and aligned with the slope of the second face of the current interrupting portion.
 10. The by-pass switch of claim 9 wherein the second face of the current interrupting portion and the beveled edge of the sliding portion has a slope of approximately 5* with respect to the second face of the sliding portion of the contact structure. 